[1.7.x] Fixed #22443 -- Document smtp.EmailBackend.timeout default value

Django 1.7 introduces a new timeout parameter for the SMTP email backend:
http://docs.djangoproject.com/en/dev/topics/email/#django.core.mail.backends.smtp.EmailBackend.timeout

However, there was no mention of the default value for this parameter.

Backport of 4e3d8ae6 from master.